Cathode grid assembly for electron gun



g- 13, 1964 R. c. PAULL CATHODE GRID ASSEMBLY FOR ELECTRON GUN Filed NOV. 14. 1960 INVEN TOR. KAY/vamp Cl 34ml.

of the ceramic washer.

United States Patent 3,145,318 CATHODE GRID ASSEMBLY FOR ELECTRON GUN Raymond C. Paul], Marion, Ind., assignor to Radio Corporation of America, a corporation of Delaware Filed Nov. '14, 1960, Ser. No. 68,977 4 Claims. (Cl. 313-82) This invention relates to electron guns for cathode ray tubes and particularly to the cathode-control grid assemblies thereof.

One type of electron gun commonly used in cathode ray tubes comprises a plurality of coaxial electrodes mounted in spaced relationship along a plurality of insulator support rods. The electrodes usually include a centrally apertured tubular control grid cup and a tubular cathode having one end closed and coated with electron emissive material and coaxially mounted within the grid cup. The cathode is mounted within a central aperture of a supporting ceramic washer by crimped circumferential beads in the cathode which bear against the opposite fiat faces The ceramic washer is in turn coaxially mounted Within the grid cup by retainer rings welded to the inner cylindrical wall of the grid cup. The cathodeis thus coaxially secured within the grid cup with its emissive coating adjacent the central aperture of the grid cup.

Although such a cathode-control grid assembly is generally satisfactory, this assembly has certain disadvantages and presents some problems. A ceramic washer, which is costly, has the added disadvantage of providing a relatively massive part which acts as a temporary heat sink. This latter characteristic causes the ceramic washer to temporarily absorb heat from the cathode until the ceramic {washer reaches a certain equilibrium temperature.

This action results in slow cathode warm up.

Where ceramic support washers are used, it has also I been a problem to prevent deposition of sublimated cathceramic from cathode deposits.

In assemblies Where the cathode is crimped to the ceramic washer as described above, chippingof ceramics is a problem. Moreover, mounting of the cathode within the ceramic by circumferentially-crimped beads has not been entirely satisfactory in that the degree of contact between the cathode and the ceramic washer, and hence the thermal conductivity therebetween, is not uniform from assembly to assembly. Thus, for a given amount of input heater power, the cathode operating temperatures of various electron guns may vary to an unacceptable degree from the norm.

Furthermore, where the cathode is mounted in a ceramic washer, electrical contact to thecathode is made by welding a tab directly to the cathode. This is also undesirable in that it results in: an additional heat sink whichlowers the temperature efiiciency of the cathode; the necessity of direct welding to the cathode after applying the emissive coating thereto which might result in damage to the cathode; and making more diflicult the use of electrical space measuring techniques for adjusting cathode-to-control grid spacing.

Various cathode-control grid assemblies have been proposed which avoid the use of ceramic mounting washers. However, these structures or" the prior arthave not provided a satisfactory solution to the electrical leakage problem caused by cathode sublimation and have, moreover,

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been expensive and subject to microphonics because of their lack of ruggedness.

It is therefore an object of my invention to provide an electron gun having a new and improved cathode-control grid assembly which avoids the problems described above.

It is also an object of my invention to provide a new and improved cathode-control grid assembly wherein the cathode and control grid are mounted independently of each other and which assembly dispenses with the use of a ceramic support washer.

Another object of my invention is the provision of a new and improved cathode-control grid assembly which prevents the formation of harmful electrical leakage paths caused by sublimation of cathode material.

It is a further object of my invention to provide a new and improved cathode-control grid assembly which is rugged, and exhibits improved uniformity of temperature characteristics from assembly to assembly.

Briefly, according to my invention, a tubular cathode having an end wall coated with electron emissive material is supported coaxially within a tubular cathode shield by a cathode support ring between the cathode and the shield. The shield is, in turn, mounted on an insulator support rod. A tubular centrally apertured control grid cup is coaxially telescoped over the shield and the emissive end of the cathode to an extent suiiicient to substantially conceal the external surface of the cathode. The control grid is mounted on the insulator support rods independently of the cathode.

In the drawings:

FIG. 1 is a longitudinal section of portions of a cathode ray tube containing an electron gun according to this invention; and

FIG. 2 is an enlarged longitudinal section of the cathode-control grid assembly of the electron gun of FIG. 1.

FIG. 1 shows an electron gun 6 disposed within a neck 8 of a cathode ray tube envelope. The gun 6 and neck 8 may, for example, form a portion of a cathode ray tube which includes an envelope funnel section 10 attached to the neck 8, a faceplate 12 closing the large end of the funnel remote from the neck 8, and a phosphor screen 13 on the internal surface of the faceplate. The neck 3 is closed at one end by a stem 14 which includes a pinrality of stiff lead-ins 15 sealed therethrough for supplying suitable potentials to the electrodes of the gun 6.

The electron gun 6 comprises a plurality of circularly symmetrical, for example tubular, electrodes mounted in coaxial spaced relationship along a plurality of glass insulator support rods 16. The electrodes include a cathode structure 17, control 'grid 18, screen grid 21), a first lens electrode 22, a focus electrode 24, and a second lens elec- Each of these electrodes is independently mounted on the glass rods 16 by pairs of U shaped elecwelded to their respective electrodes and embedded in the glass rods 16. A coiled heater 33 is disposed within the cathode and is supported on a pair of straps 34 which are likewise embedded in the glass rods 16.

The electron gun 6 is supported at one end within the neck 3 on some of the stiif lead-ins 15, and at the other end by a plurality ofv spring snubbers 35. A ring-type getter 36 is mounted coaxially with the electrodes of the gun 6 on the second lens electrode 26.

FIG. 2 illustrates in greater detail the cathode and control grid assembly of the electron gun 6 of FIG. 1, which assembly is made in accordance with this invention. The

control grid 18 comprises a tubular cup-like electrode having an end wall 4% with a small central aperture 42 to the bights of the pair of electrode mounting elements 28, the ends of which are embedded in the glass support rods 16.

The cathode structure 17 comprises a tubular cathode 46 closed at one end with a transverse wall 48 which is coated on the external surface with electron emissive material 59. The tubular cathode 46 is disposed coaxially Within a tubular cathode shield 52 which includes an outwardly extending radial flange 54 at the end thereof remote from the emissive end of the cathode 46. The cathode 46 is supported within the cathode shield 52 by a cathode support ring 56 fixed therebetween. The support ring 56 includes an inner cylindrical portion 53 welded to the external cylindrical surface of the cathode 46 at its end opposite the emissive end thereof, and an outer cylindrical section 60 welded to the inner cylindrical surface of the cathode shield 52. As shown in the drawing, the cathode support ring 56 may be welded to the shield 52 adjacent its flanged end.

The cathode shield 52 and the cathode 46 are mounted on the glass support rods 16 independently of the control grid 18 by the pair of U-shaped electrode mounting elements 27, the bights of which are Welded to the shield 52 and the ends of which are embedded in the glass rods 16.

The cathode 46 and cathode shield 52 are disposed coaxially within the control grid cup 18 with the emissive end 50 of the cathode adjacent the central aperture 42 of the grid cup 18. The tubular wall 44 of the grid cup 18 extends back along the cathode 46 and shield 52 to adjacent the electrode mounting elements 27 which support the cathode and shield.

The amount of longitudinal overlap of the tubular wall 44 of the grid cup and the cathode shield 52 is such that the external tubular surface of the cathode 46 is substantially concealed from along line-of-sight paths extending out of the open end of the grid cup 18. Stated in other words, the shield 52 extends sufllciently into the grid cup 18 that it intersects a straight line from the edge 62 of the closed end of the cathode 45 to the edge 64 of the open end of the grid cup 18. As a result of this concealment of the cathode 46, material evaporated from the cathode during operation thereof deposits on the inner surface of the grid cup 18 and on the cathode shield 52. However, inasmuch as evaporation of the material from the cathode travels in straight lines from the cathode, no evaporated material passesout of the open end of the grid cup and deposits on the glass insulator rods 16. As a result, the insulative mounting between the cathode and control grid is completely shadowed from possible deposition of conductive coating which might contribute to high resistance electrical leakage between these two electrodes.

In the cathode-control grid assembly according to the invention, good heat isolation of the cathode is provided by making the cathode support ring 56 of thin metal. Moreover, the cathode shield 52 is provided with an internal surface of high heat reflectivity. Thus, heat dissipation by radiation from the cathode is minimized, increasing cathode heating efliciency. By virtue of the independent mounting of the cathode 46 on the glass support rods 16, electrical connection may be provided to the cathode by afixing one of the lead-ins to one of the electrode mounting elements 27. Thus, the necessity of a connection tab fixed directly to the cathode 46 is avoided, and consequently, possible excessive heat loss from the cathode to the tab is avoided.

The structure of the present invention also facilitates the use of electrical measuring techniques for adjusting the cathode-to-control grid spacing. To this end, the structure of FIG. 2 can be fabricated by first fixing the cathode 46 to the cathode support ring 56 separately from the cathode shield 52, and mounting the control grid 18 and the cathode shield 52 on the glass support rods 16. Thecathode 46, together with its support ring 56, may then be tclescoped within the cathode shield 52.

When a proper spacing of the emissive coating 50 from the end wall 40 of the grid cup 18 is reached, the cathode support ring 56 is welded to the cathode shield 52. In accordance with known techniques, electrical measurement of the spacing between the cathode 46 and the grid cup 18 may be by a capacitance bridge technique wherein the capacity between the cathode 46 and either the screen grid 20 or the control grid 18 is measured.

Longitudinal spacing of the cathode shield 52 within the grid cup 18 is not nearly as critical as is the cathodeto-control grid spacing. Hence, the spacing adjustment of the shield 52 in the grid cup 18 is performed by simple mechanical jigging techniques. The radial fiange 54 on the cathode shield 52 is provided for facilitating such see s- I claim:

1. An electron gun comprising an insulator support rod, a tubular cathode, a tubular cathode shield coaxially surrounding said cathode, a cathode support ring concentrically fixed between said shield and one end of said cathode, mounting means fixed to said insulator rod and to the end of said shield adjacent said one end of said cathode for supporting said cathode and said shield relative to said rod, a control grid cup disposed coaxially with said cathode and shield with the other ends of said cathode and shield extending thereinto, and mounting means fixed to said grid cup and said insulator rod for supporting said grid cup relative to but independently of said cathode and said shield.

2. An electron gun comprising an insulator support rod, a tubular cathode, a tubular cathode shield coaxilly surrounding said cathode, a cathode support ring concentrically fixed between adjacent ends of said cathode and said shield, mounting means fixed to said adjacent end of said shield and to said insulator rod for supporting said cathode and said shield relative to said rod, a control grid cup disposed coaxially with said cathode and shield with the other ends of said cathode and shield extending thereinto a distance suflicient to substantially conceal the external surface of said cathode, and mounting means fixed to said grid cup and to said insulator rod for supporting said grid cup relative to but independently of said cathode and said shield, said cathode, support ring, shield, and grid cup being so disposed that said cathode is axially slidable relative to said grid cup prior to said support ring being fixed to both said cathode and said shield.

3. In an electron gun comprising a plurality of circularly symmetric electrodes in coaxial spaced relation, a glass rod on which said electrodes are mounted, a tubular cathode having an end wall coated with electron emissive material, a tubular cathode shield into which said cathode is coaxially disposed, the coated end of said cathode extending beyond the adjacent end of said shield, a cathode support ring concentrically fixed to the other end of said cathode and to said shield to support said cathode from said shield, first electrode mounting means embedded in said glass rod and fixed to said shield adjacent its end opposite the end thereof adjacent the coated end of said cathode, a control grid cup having an apertured end wall and a tubular wall telescoped over the coated end of said cathode and the end of said shield adjacent thereto, the tubular Wall of said grid cup extending part way back said shield to adjacent said first electrode mounting means, and second electrode mount ing means embedded in said glass rod and fixed to the tubular wall of said grid cup.

4. An electron gun cathode-control grid assembly comprising a glass support rod, a control grid cup having a tubular Wall and a centrally apertured end wall, electrode mounting means welded to said tubular wall and embedded in said glass rod to support said grid cup relative to said glass rod, a tubular cathode having at one end a closure wall coated with electron emissive material, a tubular cathode shield into which said cathode is telescoped, the coated end of said cathode being disposed outside of said shield beyond the adjacent end thereof, a cathode support ring concentric with said cathode and said shield and having an inner cylindrical section fixed to the outer cylindrical surface of said cathode at the other end thereof and an outer cylindrical section fixed to the inner cylindrical surface of said shield to support said cathode and said shield relative to each other, said cathode and shield being coaxial with said grid cup with the coated end of said cathode and said shield extending thereinto a distance suflicient to substantially conceal the external tubular surface of said cathode from along line-of-sight paths extending out of the open end of the grid cup, and electrode mounting means embedded in said glass rod and Welded to said shield adjacent the end thereof op posite the end thereof adjacent the coated end of said cathode to support said cathode relative to said glass rod independently of said grid cup, said cathode being axially slidable relative to said grid cup prior to said support ring being fixed to both said cathode and said shield.

References Cited in the file of this patent UNITED STATES PATENTS 2,081,942 Lubcke a- June 1, 1937 2,383,751 Spangenberg Aug. 28, 1945 2,510,267 Tolson June 6, 1950 2,914,694 Chin Nov. 24, 1959 FOREIGN PATENTS 827,557 Great Britain Feb. 10, 1960 

1. AN ELECTRON GUN COMPRISING AN INSULATOR SUPPORT ROD, A TUBULAR CATHODE, A TUBULAR CATHODE SHIELD COAXIALLY SURROUNDING SAID CATHODE, A CATHODE SUPPORT RING CONCENTRICALLY FIXED BETWEEN SAID SHIELD AND ONE END OF SAID CATHODE, MOUNTING MEANS FIXED TO SAID INSULATOR ROD AND TO THE END OF SAID SHIELD ADJACENT SAID ONE END OF SAID CATHODE FOR SUPPORTING SAID CATHODE AND SAID SHIELD RELATIVE TO SAID ROD, A CONTROL GRID CUP DISPOSED COAXIALLY WITH SAID CATHODE AND SHIELD WITH THE OTHER ENDS OF SAID CATHODE AND SHIELD EXTENDING THEREINTO, AND MOUNTING MEANS FIXED TO SAID GRID CUP AND SAID INSULATOR ROD FOR SUP- 